There are two main shortcomings to the current techniques for using an acoustic drum to activate the sound programs of synthetic drum machines: tracking sensitivity and mechanical reliability. Drum synthesizers are designed to be triggered by a device called a "pad." Pads are very different from acoustic drums in that they are highly damped, solid devices containing a transducer capable of detecting the force of impact of a drum stick. Because of its highly damped nature, the duration of a pad's output signal is very short, i.e. on the order of 4-8 milliseconds. Since a pad is solid, with no resonating chamber, it is incapable of producing any appreciable acoustic sound by itself. A drum synthesizer is designed for a 4-8 millisecond trigger input to activate its sound output.
In order to produce an electronic trigger signal from an acoustic drum, a transducer has been attached to the drum to detect the force of impact of a drum stick on the drum head. However, because of the drum's tendency to resonate after the initial impact the trigger output is significantly longer than 4-8 milliseconds. Also a drum is not designed to have transducers attached to it, and the current methods of attaching the transducer are often mechanically unreliable.
There are three basic methods of mounting transducers on an acoustic drum: air coupling including internal mounting, head mounting and shell mounting.
Air coupling consists of a transducer such as a microphone that is suspended inside or outside of the drum shell in close proximity to the drum head. This method picks up the vibrating air caused by the movement of the drum head. Internal mounting of a microphone often involves extensive modification of the acoustic drum and constant monitoring to ensure mechanical reliability. The duration of the signal produced is very long, and the microphone cannot discriminate between drum sound and background noise.
Head mounting usually involves gluing or taping a transducer to the drum head. The main disadvantage of this technique is the damping of the natural drum sound, adversely affecting the acoustic characteristics of the drum sound. Also, the adhesive used typically cannot withstand the constant vibration of the drum head, and the transducer eventually falls off.
In shell mounting, a transducer is glued or screwed to the drum shell, and the transducer can be either inside or outside of the drum. Shell mounting is mechanically more reliable, and results in the shortest signal duration of the three mounting methods. It is, however, not as sensitive to head movement as it is to shell movement. The shell will vibrate with the drum head, and the signal from the shell transducer will have charcteristics that are undesirable for triggering purposes. The transducer used in head and shell mounting is almost always small a piezoelectric wafer or bar less than an inch in length or diameter. The transducer is often enclosed in a small light weight plastic or metal housing which may have holes for mounting screws.
All three transducer mounting methods have similar disadvantages: dangling wires, unreliable mounting schemes and a relatively long signal duration, typically 100-400 milliseconds. The long signal duration often results in unwanted re-triggering. The most common method of overcoming this problem is to set the sensitivity threshold of the drum synthesizer to an area where the tail end of the transducer signal cannot cause re-triggering of the drum synthesizer. However, reducing the sensitivity threshold results in substantially limiting the range of force of impact of a drum stick that will trigger the drum synthesizer. For example, a drummer may be forced to play in the medium soft to medium loud range. To maximize the range of force that the drum synthesizer will track, an alternate method of reducing the signal duration is to dampen the drum head. The disadvantage here is the adverse effect on the sound quality of the acoustic drum.